The present invention relates to a high-density data recording technique, and more particularly, to a method of manufacturing a recording medium excellent in thermal stability of magnetic recording data.
Recently, magnetic recording media have been aggressively studied and developed as a possible means for recording a large amount of data. Among them, in magnetic recording media for use in computer hard disk drives, tendency toward a high areal recording density has been tremendously accelerated.
Now used as the recording medium is the recording system called "longitudinal magnetic recording". The longitudinal magnetic recording is a recording method for recording signals in an in-plane direction of the recording film by directing a magnetization vector to the in-plane direction. However, to attain further higher-density recording, attention has been attracted to "perpendicular magnetic recording", which is a recording method for recording signals in a perpendicular direction of the recording film by directing the magnetization vector to the perpendicular direction (see S. Iwasaki and Y. Nakamura; IEEE Trans. Magn., vol. MAG-13, pp 1272-1277, 1977).
In either recording system, a Co--Cr system alloy is principally used as a magnetic recording material for a recording layer. In this case, the crystal orientation of the Co--Cr system alloy is controlled by a material, a crystal orientation, or a lattice constant of its underlayer placed immediately thereunder. It is therefore possible to control a direction of an easy axis of magnetization responsible for defining a direction of the magnetization vector. Today, using such a technique, a longitudinal magnetic recording medium or a perpendicular magnetic recording medium using the Co--Cr system alloy is manufactured.
However, when the Co--Cr system alloy is employed in the longitudinal magnetic recording, a phenomenon called "thermal relaxation" occurs (reported by P. L. Lu and S. H. Charap (IEEE Trans. Magn., vol. 30, 4230 (1994))). The "thermal relaxation" is a phenomenon where recorded data is deteriorated during long term storage, as the recording density increases.
In contrast, the perpendicular magnetic recording is superior to the longitudinal magnetic recording in consideration of not only a possibility of the high-density recording but also thermal relaxation (reported by S. Iwasaki, K. Ouchi and N. Honda, IEEE, vol. 32, 3795 (1996)).
However, as shown in a computer simulation reported by Jiang, Muraoka, Takawa, and Nakamura (Jpn. J. Appl. Phys. Vol. 21, 293, (1997)), the thermal relaxation also occurs in the perpendicular magnetic recording medium.
The thermal relaxation remarkably occurs as a ratio of magnetic energy (K.sub.u .times.v) of a magnetic grain contained in a magnetic material to thermal energy (k.sub.B .times.T) from the circumstance surrounding the magnetic material decreases. EQU (K.sub.u .times.v)/(k.sub.B .times.T) (Equation 2)
Where, K.sub.u is a magnetocrystalline anisotropy constant, v is magnetic grain size, k.sub.B is the Boltzmann constant, and T is an absolute temperature of the circumstance.
In other words, the thermal relaxation manifests more clearly as the recording density increases. This is because the size of magnetic grains decreases with the increase in recording density. Therefore, to increase the energy ratio, the magnetocrystalline anisotropy constant intrinsic to the recording material must be increased.
K. R. Coffey, M. A. Parker and J. K. Howard et al. (IEEE Trans. Mag., vol. 31, 2737 (1995)) form a thin film with random crystalline orientation using an ordered alloy with L1.sub.0 crystal structure to realize the longitudinal magnetic recording. According to this technique, after completion of film formation, the thin film is annealed to form an ordered phase having high magnetocrystalline anisotropy. In this case, however, the obtained perpendicular magnetization component is small. It is therefore difficult to apply this technique to the perpendicular magnetic recording medium in expectation of the high density recording.
Furthermore, T. Suzuki, N. Honda and K. Ouchi (J. Magn. Soc. Jpn., 21-S2, 177 (1997) have suggested a sputter deposition method and a layer structure in order to attain the perpendicular crystalline orientation of the ordered alloy with L1.sub.0 crystal structure. However, a sufficient perpendicular anisotropy cannot be imparted by this technique. Furthermore, since the film used in the magnetic recording medium in practice is as thin as about 50 nm or less, it does not exhibit a hard magnetic property. Therefore, such a film cannot be used as a magnetic recording medium as long as it is maintained in this state.
On the other hand, as a method of attaining perpendicular anisotropy of the ordered alloy thin film with L1.sub.0 crystal structure, a sputter deposition method using an MgO single crystalline substrate is disclosed by M. Watanabe and M. Homma (Jpn. J. Appl. Phys. Vol. 35, L1264 (1996)). However, since the MgO single crystalline substrate is expensive, it is difficult to industrially use it as the substrate for use in a hard disk. Furthermore, since processing is performed at as high a temperature as 600.degree. C., a glass or an Al substrate for a hard disk cannot be used. Moreover, the magnetic property obtained by this method is a so-called "wall motion mechanism in its magnetization reversal process". Even if signals are recorded on this thin film, a medium noise would be too large to obtain a sufficient signal to noise ratio. Therefore, it is easily predicted that the film obtained by this method is not suitable as the magnetic recording medium.
From the foregoing, it has been strongly desired to develop a method of forming a perpendicular magnetic anisotropic thin film having a high magnetocrystalline anisotropy and a small magnetic domain structure (which means that medium noise is low) by using the industrially applicable hard disk substrate.